Objectives: Recurrent aphthous stomatitis (RAS) is a common ulcerative disease of the oral mucosa. Methylenetetrahydrofolate reductase (MTHFR) gene variants are associated with thrombophilia and vasculopathy that may result in oral ulceration. Oral ulcers are also the most common feature of Behcet's disease (BD). Association of MTHFR gene C677T mutation with BD has been reported in different populations. The aim of the present study was to investigate the possible association between MTHFR gene C677T mutation and RAS and evaluate if there was an association with clinical features in a relatively large cohort of Turkish patients. Materials and methods: The study included 188 patients affected by RAS and 200 healthy controls. Genomic DNA was isolated and genotyped using polymerase chain reaction (PCR)-based restriction fragment length polymorphism (RFLP) assay for the MTHFR gene C677T mutation. Results: The genotype and allele frequencies of C677T mutation showed statistically significant differences between RAS patients and controls (p = 0.002 and p = 0.0004, respectively). After stratifying RAS patients according to clinical characteristics of oral ulcers, a significant association was observed between C677T mutation and number of oral ulcers of RAS patients (p = 0.006). Conclusions: As a result, a high association between MTHFR gene C677T mutation and RAS was observed in the present study. Also number of oral ulcers was found to be associated with MTHFR C677T mutation in RAS patients. Clinical Relevance: If our observation can be substantiated with further studies, evaluation for MTHFR mutations and perhaps folate supplementation may become necessary in selected patients. © 2013 Springer-Verlag Berlin Heidelberg.